Bug Description

RANDOM-INTEGER-EXTRA-BITS=10 may not be large enough for the RANDOM
RNG to be high quality near RANDOM-FIXNUM-MAX; it looks as though
the mean of the distribution can be systematically O(0.1%) wrong.
Just increasing R-I-E-B is probably not a good solution, since
it would decrease efficiency more than is probably necessary. Perhaps
using some sort of accept/reject method would be better.

I have just committed a reworked version of the patch from #2.
Differences:
- renamed "random-bignum.lisp" to "bignum-random.lisp"
- renamed RANDOM-CHUNK-LENGTH to N-RANDOM-CHUNK-BITS
- rewrote various comments to explain things better;
use "accept-reject" instead of "accept/reject" throughout
- corrected a factual error in the commit message and a comment:
%random-fixnum uses only one random chunk when sufficient
- added a NEWS entry
- added credits to the commit message